MK: You've sort of stumbled on one of my pet high horses with this. I've quoted you just as a jumping off point, but the following is not specifically directed at you. You're probably well aware of all this stuff...
Originally Posted by MKtheater
You know this is a great point. Many processors say that MV 0 should calibrated reference levels but movies vary all the time in how loud they are. Many movies with the volume on 0dB MV is over reference level.
That's because reference calibration is only an "anchor" - the ship (movie) is free to drift around all over the place, within the limits of the anchor chain!
Reference calibration is really just a simple, reproducible method of setting up the total gain structure of the system [system = source > processor > amp > speakers > receptor (ears/meter)]. The notion that "reference" is defined
by particular playback
SPL's is incorrect. This inaccuracy is probably fuelled by the "lay-person friendly" descriptions on various websites and (I'm afraid to say) the authoritative sounding, but incorrect posts of some senior, well-respected forum members.
The accurate definition is one most are probably all familiar with: It's the input voltage required such that the total electro-accoustic gain of the production or playback chain, when fed a -20dBFS band limited (500Hz - 2000Hz) pink noise signal, produces 85dBSPL (C-weighted, slow reading) from each main channel, when measured at the listening position. This is a good discussion by the pros: http://abluesky.com/support/blue-sky-calibration-test-files/
. You will note that they mention 20dB of headroom, but nowhere do they mention 105dBSPL as a maximum output level.
What's important to realise is that this gain matches reference level only for that particular test signal
and measurement method. Beyond that, all bets are off! Extrapolating this to specific playback SPL's (eg 105dBSPL mains; 115dBSPL LFE) of program material (i.e. movies) is drawing too long a bow because the SPL of any other signal will be effected by various other equipment and room factors that haven't been (can't be) taken into account in the calibration. For example, frequencies outside
the bandwidth of the test signal will interact with room modes and will cause differences in actual playback SPL's between properly calibrated systems. In addition, even within
the bandwidth of the test signal there will be differences in playback SPL due to the measurement method. The slow setting on the meter averages SPL's over a one (I think?) second period, so if program peaks significantly less than one second in duration are reinforced by a room mode, they will not have been accounted for in the calibration process either!
Those interested just need to remember the theory of reference level calibration and not extrapolate conclusions about precise program playback SPL's. In other words, if your system is properly calibrated to reference, the best you could say is that your favourite Blu-ray movie will play at roughly
the same SPL at a given MV setting as the next blokes properly calibrated system. (This is why I emphasised roughly
in post #733.
Another aspect to all this is the presumption that every last drop of (digital) headroom is utilised by the movie's soundtrack creators during loud program peaks. That is, use of 0dBFS signals. As far as I'm aware they don't... peak signals of -4dBFS in the main and LFE channels are more typical in Dolby Digital soundtracks. I did read an AVS thread (can find it now) where a member related that when THX are supervising a soundtrack, they generally don't like see signals hotter than -6dBFS, and anything hotter than -4dBFS is a definite no-no. So again, the often quoted "105dB for mains; 115dB for LFE at reference" is looking a bit arbitrary.
Originally Posted by MKtheater
A simple way to check is to run just your center channel at reference and play a very loud movie and set your MV to whatever peaks at 105 dBs.
That would be an interesting exercise, but you would be measuring the movie as well as your rooms effect on it, yes? Plus, you would have gathered, I don't think there's anything magic about 105dBSPL.